Polysilicone flexible foams are commonly made by the mixing together of mutually interreactive ingredients in the presence of a catalyst, with the gas evolved as a reaction product acting as the blowing agent. A typical example of such a formulation and process is described in U.S. Pat. No. 4,189,545 Modic. Such formulations contain two distinct parts, which are kept separate from one another until the foam is to be produced. The two parts react together relatively rapidly on contact, with the formation of high molecular weight polymer and generation of gas to cause foam expansion. The high molecular weight polymer then proceeds to gel and then to cure, all of these chemical and physical changes being capable of taking place at the mixing temperature (commonly room temperature), but preferably conducted at slightly elevated temperatures. The gelling process takes place rapidly, after which no further foam expansion can take place. It is quite common to include a retardant in the formulation for the specific purpose of prolonging the gel time, so that foam expansion may continue to its full extent. Curing is a slower process, and whilst it will occur naturally, at room temperature, after the gelling process is complete, it is common to warm the foam in an oven to complete the curing process.
Thus there take place, successively and rapidly following the contact of the reactive ingredients, the processes of chemical reaction, high molecular weight silicone polymer formation, gas generation, foam formation and expansion, and gelling. The quality of the end product foam, in terms of its open or closed cell nature, the size and uniformity of the cell structure, the density of the resulting foam and other properties, depends at least in part upon the homogeneity of the mixture at the time the foaming takes place, which in turn depends upon the thoroughness of the mixing of the components prior to foaming, and the maintenance of even temperature throughout the mixture.
However, the foamable composition ingredients are normally in the form of two dissimilarly viscous liquids and are not easily mixed into a sufficiently homogeneous composition before the chemical reaction to form the foam occurs. In the normal system, e.g. as disclosed in the aforementioned Modic Patent, the ingredients of the foamable composition are pre-combined into a part A and a part B, the constituents of each part being unreactive towards one another. In part A, there is a siloxane polymer, commonly a polydiorganosiloxane with vinyl terminal groups, filler, water and variable amounts of reactive diluent, plus a complex platinum based catalyst. In part B, there is a polydiorganosiloxane polymer containing hydride groups. Both parts are viscous liquids, although the problem is further complicated by the fact that one part is much more viscous than the other. Substantially immediately upon contact, the polymers and water react together under the influence of the catalyst, to generate hydrogen gas for blowing purposes and to form the high molecular weight polydiorganosiloxane curable to a high strength flexible foam material. Foaming occurs as the gas is released and expands. It is difficult to achieve the necessary homogeneous mixing of the ingredients to allow preparation of good quality homogeneous foam, along with even distribution of the fire retardant filler therethrough, in the time available before the mixture sets up and gels, especially on a commercial production scale. The time interval available for this mixing is at most about three minutes, and commonly as short as one minute or less. The reaction is sensitive to heat, and some heat should be supplied to achieve the best foams. However excessive provision of heat leads to uncontrollably fast reaction. Any heat should be supplied evenly and homogeneously to the reaction mixture, since the development of temperature inequities or hot-spots in the reaction mixture will lead to inhomogeneous, poor quality foam. Heat primarily affects gas generation.